There is known a work machine, such as a hydraulic excavator, which includes a periphery monitoring device for detecting an obstacle, such as a human or an object, present in a periphery of the work machine, and issuing a warning to an operator of the work machine to prevent a collision between the work machine and the obstacle. In an actual work site, however, an operator often intentionally operates a work machine even when recognizing a worker is working near the work machine. In this environment, a warning is issued from the periphery monitoring device even when the worker present near the work machine is recognized.
Patent Document 1 describes one of devices attempting to solve the foregoing problem. The device described in Patent Document 1 determines whether or not an obstacle present in a periphery of a work machine is a human through image processing, and selects a warning level based on a state of the work machine and a positional relationship with the obstacle to reduce unnecessary warnings. A device described in Patent Document 2 determines whether or not an operator visually recognizes an obstacle based on a visual line of the operator, and issues a warning based on the determination to prevent excessive warnings.